1. Field of the Invention
The present invention relates to a compact, wide angle internal focusing lens suitable for video cameras, electronic still cameras, and the like.
2. Description of the Related Art
In general, imaging lens focusing methods include a front lens scheme in which a first lens group, which is disposed farthest on the object side of the optical system, is moved; an internal focusing scheme in which a second lens group or subsequent lens group disposed farther on the image side of the optical system than the first group, is moved; and a real focusing scheme. Compared to the lenses used for the front lens scheme, the effective optical diameter of the first lens group used in the internal focusing scheme and the real focusing scheme can be made smaller, contributing to the advantage of a smaller dimension of the optical system in terms of diameter. Internal focusing and real focusing lenses perform focusing by moving a lens group of a relatively lighter weight and consequently, when used in an auto-focusing camera, which has become mainstream, offer a further advantage of achieving stable auto-focusing. Thus, many internal focusing and real focusing lenses having such characteristics have been proposed (see, for example, Japanese Patent Application Laid-Open Publication No. 2009-237542, Japanese Patent Nos. 3950571 and 3445554).
The optical system recited in Japanese Patent Application Laid-Open Publication No. 2009-237542 includes a front group and a rear group sandwiching the aperture stop and performs focusing by moving the rear group. However, since the rear group is configured by approximately 3 lenses, the weight of the focusing group is not reduced sufficiently, making high speed focusing difficult to carry out smoothly.
The optical system recited in Japanese Patent No. 3950571 includes a focusing group configured by approximately 1 lens and therefore, compared to the optical system disclosed in Japanese Patent Application Laid-Open Publication No. 2009-237542, facilitates a lighter focusing group. Nonetheless, the optical system recited in Japanese Patent No. 3950571 does not achieve a sufficient wide angle view. In particular, at the converted focal length for a 35 mm camera, the angle of view for an area in the vicinity of 45 mm becomes narrow.
Similar to the optical system recited in Japanese Patent No. 3950571, the angle of view of the optical system recited in U.S. Pat. No. 3,445,554 is difficult to increase. The optical system recited in Japanese Patent No. 3950571 has a further problem in that the size is not sufficiently reduced.
As described, among conventional internal focusing and real focusing lenses, including the cited optical systems, no lens simultaneously achieves a compact optical system, increased wide angle view, and a focusing group of a sufficiently light weight.